To cite sphet in publications use:

Bivand R, Millo G, Piras G (2021). “A Review of Software for Spatial Econometrics in R.” Mathematics, 9(11). doi:10.3390/math9111276, https://www.mdpi.com/2227-7390/9/11/1276.

Piras G (2010). “sphet: Spatial Models with Heteroskedastic Innovations in R.” Journal of Statistical Software, 35(1), 1–21. doi:10.18637/jss.v035.i01.

Bivand R, Piras G (2015). “Comparing Implementations of Estimation Methods for Spatial Econometrics.” Journal of Statistical Software, 63(18), 1–36. doi:10.18637/jss.v063.i18.

Corresponding BibTeX entries:

  @Article{,
    title = {A Review of Software for Spatial Econometrics in {R}},
    author = {Roger Bivand and Giovanni Millo and Gianfranco Piras},
    journal = {Mathematics},
    year = {2021},
    volume = {9},
    number = {11},
    url = {https://www.mdpi.com/2227-7390/9/11/1276},
    doi = {10.3390/math9111276},
  }

  @Article{,
    title = {{sphet}: Spatial Models with Heteroskedastic Innovations
      in {R}},
    author = {Gianfranco Piras},
    journal = {Journal of Statistical Software},
    year = {2010},
    volume = {35},
    number = {1},
    pages = {1--21},
    doi = {10.18637/jss.v035.i01},
  }

  @Article{,
    title = {Comparing Implementations of Estimation Methods for
      Spatial Econometrics},
    author = {Roger Bivand and Gianfranco Piras},
    journal = {Journal of Statistical Software},
    year = {2015},
    volume = {63},
    number = {18},
    pages = {1--36},
    doi = {10.18637/jss.v063.i18},
  }